Modern level control arrangements have an overpressure function and a residual pressure holding function. An overpressure function is understood to be a function wherein the air pressure source is connected to the atmosphere during filling of one of the air springs of the level control arrangement when the air pressure source exceeds a pregiven pumping pressure. In this way, it is ensured that no overpressure can be built up in the air springs which could damage the same. A residual pressure holding function is a function wherein each air spring of the level control arrangement can be deflated only to a specific pregiven residual pressure. In this way, it is ensured that the air pressure in the air springs does not drop below this pregiven residual pressure so that a specific support capability of the air spring is always retained.
A compressor is sold by WABCO (Westinghouse Fahrzeugbremsen GmbH) having the product number 415 403-1020 or 415 403-1040. This compressor has a pneumatically controllable first directional valve wherein the overpressure function and the residual pressure holding function are integrated. The overpressure function is ensured in that, when filling an air spring with the aid of the compressor, the second piston is charged by the air pressure of the compressor and opens to the atmosphere against the return spring force of the second spring when the compressor exceeds a pregiven pumping pressure. The compressor then communicates with the atmosphere and pumps only into the atmosphere.
To deflate an air spring, the first piston and the second piston are each lifted from their seats against the return force of the first spring and the return force of the second spring by the air pressure in the air spring and the air spring is then connected to the atmosphere. The return spring force of the first spring is so adjusted that this force presses the first piston back onto the seat when the residual pressure in the air spring is reached thereby blocking the connection of the air spring to the atmosphere. A further deflation of the air spring below the residual pressure is then no longer possible.
The pneumatically controllable directional valve is controlled for deflating an air spring as will be explained below.
A control line is conducted via a controllable additional directional valve disposed between the air springs and the pneumatically controllable directional valve. This control line is connected to the control chamber of the pneumatically controllable directional valve which then is charged with the air pressure of an air spring. As a consequence, the first piston and the second piston are lifted from their respective seats against the return forces of the first and second springs. The venting line is likewise guided via the controllable second directional valve into the control chamber. The control chamber includes an inlet opening into which the control line and the venting line open. In addition to the inlet opening, the control chamber includes an outlet opening from which the venting line extends and is connected to the air dryer of the level control arrangement. For venting an air spring, the air flows from the air spring through the controllable second directional valve and through the control chamber into the air dryer and, from there, into the atmosphere via an outlet of the pneumatic controllable directional valve.
The pneumatic controllable directional valve which is known from the compressor of WABCO, has a simple compact configuration but the overpressure function and the residual pressure function are integrated therein. It is, however, noted that, to deflate an air spring, there is a continuous flow through the control chamber of the pneumatically controllable directional valve. A throttle is provided in the outlet opening of the control chamber in order to avoid a large drop of the static air pressure in the control chamber. This throttle greatly reduces the flow speed of the air to be vented. For this purpose, the throttle has to have a very small flow cross section so that a rapid deflation of the air spring and therefore a desired rapid lowering of the vehicle (for example, when the vehicle is at standstill) is not possible.
It is an object of the invention to provide a level control arrangement having a pneumatically controllable directional valve which contains an overpressure function and a residual pressure holding function and which makes a rapid deflation or venting of the air springs possible.
The level control arrangement of the invention is for a vehicle and includes: a plurality of air springs mounted on the vehicle; a pressurized air source for supplying pressurized air; an air dryer connected to the pressurized air source; valve means for connecting the pressurized air source to the air springs via the air dryer for filling the air springs; a pneumatically controllable first directional valve for connecting the pressurized air source with the atmosphere when a pregiven pumping pressure is exceeded; a venting line providing a path via which the air springs can communicate with the atmosphere for releasing pressurized air thereinto from the air springs; the venting line being lead through the first directional valve and the air dryer; the first directional valve having a pneumatic control input; a control line leading to the pneumatic control input and being connectable to at least one of the air springs so as to permit the pneumatic control input to be charged with pressurized air of the at least one of the air springs; a controllable second directional valve connected in the control line between the first directional valve and the air springs and switchable to connect and disconnect the first directional valve from the air springs; the first directional valve including: a housing; a first piston displaceably mounted in the housing for movement between a first position and a second position and the first piston having a catch formed thereon; a first seat on which the first piston sits when in the first position; a first spring for resiliently biasing the first piston against the first seat in the first position; the first piston being liftable from the first seat against the return force of the first spring when the pneumatic control input is charged with the air pressure of at least one of the air springs; a second seat connectable to the atmosphere; a second piston displaceably mounted in the housing; a second spring for resiliently biasing the second piston against the second seat for blocking a connection to the atmosphere via the second seat; the second piston being operatively connected to the first piston and having an end face chargeable with pressurized air from the pressurized air source and from the venting line; the second piston being lifted from the second seat against the return force of the second spring via the catch when the first piston is lifted from the first seat or by the pressurized air from the pressurized air source when the pressurized air from the pressurized air source exceeds a pregiven pumping pressure and then at least one of the air springs and/or the pressurized air source is connected with the atmosphere; the venting line being guided through the first directional valve separately from the control line; and, the venting line being blocked by the first piston when the first piston is seated on the first valve seat and cleared when the first piston has lifted from the first seat so that air from at least one of the air springs can be discharged into the atmosphere.
The advantages achieved with the invention are especially that a rapid deflation of the air spring is possible via the pneumatically controllable directional valve because the venting line is guided through the directional valve separately from the control line and therefore a throttle to reduce the flow velocity in the venting line to maintain the static air pressure in the control line or in the control chamber is not necessary.
A further advantage of the invention will be understood when one considers that, in the basic state of the control system (that is, when the air springs are neither filled nor deflated), the air springs should be separated from the air dryer so that no air from the air springs can flow to the air dryer. In this case, it is possible to arrange a pressure sensor between the air springs and the air dryer with which every air spring can be connected for pressure measurement without air flowing out of the air spring into the air dryer which would lead to an unwanted pressure loss in the air spring.
The separation of the air springs from the air dryer is achieved in accordance with the invention in that the venting line is blocked in the pneumatically controllable first directional valve by the first piston when this piston is disposed on its seat. Because of this situation, the venting line is guided directly from the air springs to the pneumatically controllable first directional valve without passing through the second controllable directional valve. In this way, a rapid deflation of only two or more air springs of the level control arrangement is possible simultaneously because a large air flow can be guided through the pneumatically controllable first directional valve. (This is in contrast to the level control arrangement known from WABCO, wherein the venting line is guided via the second controllable directional valve in order to separate the air springs from the air dryer in the basic state of the level control arrangement. The second directional valve is an electrically controllable directional valve so that only small flow cross sections can be switched therein. For this reason, a rapid simultaneous deflation of the two or more air springs is not possible.)
According to another embodiment of the invention, the first directional valve includes a space formed therein into which the first piston at least partially plunges. The venting line has two component lines having respective first ends defining respective openings which open into the space; and, at least one of the openings is closed by the first piston when the piston is seated on the first seat and both of the openings are open when the first piston has lifted from the first seat so that both of the component lines are connected to each other via the space.
The advantage of this embodiment is that at least one outlet opening is closed by the seating of the first piston on the opening and can be opened by lifting the piston. In this way, a pneumatically controllable directional valve in accordance with the first embodiment of the invention is easy to produce.
According to another feature of the invention, at least one of the component lines opens into the seal seat on which the first piston sits. The first piston is held on the seal seat by the first spring. The advantage of this embodiment is that a reliable interruption of the venting line is achieved by the configuration of the seal seat when the air springs are not to be deflated.
According to another feature of the invention, the space can be an annular space or it can be a bore in the housing of the pneumatically controllable directional valve. The configuration of the space as a small area bore into which the first piston at least partially plunges affords the advantage that the area of the piston which the air passes by when deflating an air spring is especially small (the area corresponds to only the small area of the bore). For this reason, a precise control of the pneumatically controllable first directional valve via the pneumatic control input by means of the air flow in the venting line is only affected by a negligible amount. This will be explained in greater detail in the detailed description of the preferred embodiments.
In another embodiment of the invention, the first piston has a surface and defines a longitudinal axis. The housing has an inner wall adjacent the surface of the first piston and the inner wall is configured to axially guide the first piston in the housing. The venting line has two component lines lying in spaced relationship to each other with the spacing therebetween being in the direction of the longitudinal axis. The component lines have respective first ends defining respective openings which open at the surface of the first piston. The inner wall and the surface of the first piston conjointly define an interface. The first directional valve further includes a seal mounted at the interface and the seal is operatively connected to the first piston so that a connection between the component lines along the interface is blocked when the first piston is in the first position and the connection is cleared when the first piston has lifted off the first seat.
The advantage of this embodiment is that no force is applied to the first piston because of the air pressure in the venting line which force operates in the axial direction of the piston and could lift the piston from its seat so that a precise control of the pneumatically controllable first directional valve takes place exclusively via the pneumatic control input of this directional valve.
According to another embodiment of the invention, the first piston includes a slot in which an O-ring is arranged. The slot lies between the two component lines and is radially pressed between the first piston and the housing wall when the piston is disposed on its seat and lies above or below the component lines when the first piston is lifted from its seat. The advantage of this further embodiment is that the pneumatically controllable first directional valve can be manufactured in a simple manner.
According to another embodiment of the invention, the venting line includes a throttle which is mounted between the air springs and the air dryer. The advantage of this further embodiment is that the air, which is released from an air spring of the level control arrangement, is slowed by the throttle so that an especially good regeneration of the air dryer is possible. It has been shown that even one throttle having a flow cross section of 1 mm to 2 mm satisfies this function.
Preferably, the throttle is mounted in the venting line in such a manner that the air flows through this throttle exclusively during deflation or venting of an air spring. The advantage of this further embodiment is that there is no flow through the throttle during filling of an air spring and so the filling operation is not hindered. The throttle can, for example, lie in the region wherein the venting line is guided into the pneumatically controllable first directional valve in order to satisfy the above-mentioned conditions.